72 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



The central cells of the fundus glands and the cells of the pyloric 

 glands are loaded with granules. During secretion they discharge 

 their granules, those that remain being chiefly situated near the 

 lumen, leaving in each cell a clear outer zone (see fig. 20). These 

 are the cells that secrete the pepsin. Like secreting cells generally, 

 they select certain materials from the lymph that bathes them : 

 these materials are worked up by the protoplasmic activity of the 



cells into the secretion, which is then dis- 

 charged into the lumen of the gland. The 

 most important substance in a digestive 

 secretion is the ferment. In the case of a 

 gastric juice this is pepsin. We can trace 

 an intermediate step in this process by the 

 presence of the granules. The granules 

 are not, however, composed of pepsin, but 

 of a mother-substance, which is readily 

 converted into pepsin. We shall find a 

 similar ferment precursor in the cells of 

 the pancreas, and the term zymogen is 

 applied to these ferment precursors. The 

 zymogen in the gastric cells is called pep- 

 sinogen. The rennet-ferment or rennin that 

 causes the curdling of milk is distinct from 

 pepsin, 1 and is preceded by another zymo- 

 gen ; it is, however, formed by the same 

 cells. 



The parietal cells are also called oxyntic 

 cells, because they secrete the hydrochloric 

 acid of the juice. Heidenhain succeeded 

 in making in one dog a cul-de-sac of the 

 fundus, in another of the pyloric region of 

 the stomach ; the former secreted a juice 

 containing both acid and pepsin ; the latter, 

 parietal cells being absent, secreted a viscid 

 alkaline juice containing pepsin. The for- 

 mation of a free acid from the alkaline 



blood and lymph is an important but puzzling problem. There 

 is no doubt that it is formed from the chlorides of. the blood 

 and lymph, and of the chemical theories advanced as to how 

 this is done, Maly's is the most satisfactory. He considers that the 



1 The individuality of rennin has been questioned by Pawlow, who regards its 

 action as a phase of pepsin activity. 



FIG. 20. A fundus gland of simple 

 form from the bat's stomach. 

 Osmic acid preparation (Lang- 

 ley) : c, columnar epithelium 

 of the surface ; , neck of the 

 gland, with central and parietal 

 cells ; /, base occupied only 

 by principal or central cells, 

 which exhibit the granules 

 accumulate! towards the lumen 

 of the gland. 



